When someone greets a Christian whom he does not recognize

'Abdu'r-Rahman said, "Ibn 'Umar passed by a Christian who greeted him and Ibn 'Umar returned the greeting He was told that the man was a Christian. When he learned that, he went back to him and said, 'Give me back my greeting.'"

حَدَّثَنَا مُحَمَّدُ بْنُ كَثِيرٍ، قَالَ: أَخْبَرَنَا سُفْيَانُ، عَنْ أَبِي جَعْفَرٍ الْفَرَّاءِ، عَنْ عَبْدِ الرَّحْمَنِ قَالَ: مَرَّ ابْنُ عُمَرَ بِنَصْرَانيٍّ فَسَلَّمَ عَلَيْهِ، فَرَدَّ عَلَيْهِ، فَأُخْبِرَ أَنَّهُ نَصْرَانِيٌّ، فَلَمَّا عَلِمَ رَجَعَ إِلَيْهِ فَقَالَ: رُدَّ عَلَيَّ سَلامِي.